It has previously been proposed to provide an ABS in which braking pressure applied to the front wheels is separately, individually controlled and, further, the braking pressure to the rear wheels is controlled, which is so arranged that, in case of yaw torque, the braking effort being applied to the respective wheels is reduced. In one such system--see the referenced U.S. Pat. No. 4,593,955, Leiber, to which German Patent Disclosure Document DE-OS No. 34 21 700corresponds--the pressure rise on one front wheel is limited if the braking pressure to the other front wheel is controlled by the ABS. The control characteristics themselves are changed when operation of the vehicle in a curved path is sensed. Such sensing of the curved path can be obtained either by sensing response of a transverse accelerometer or by deriving a curved path signal by comparing speed differences between wheels operating at the inside of a curve and at the outside of a curve. The system of the referenced U.S. Pat. No. 4,593,955 also proposes to disable the yaw torque pressure decrease upon sensing a predetermined transverse acceleration, for example when the vehicle is in a curve.
Decreasing of yaw torques is used in vehicles with ABS control in order to prevent differences in braking effort on the front wheels of the vehicle to become too high if the coefficient of friction of the respective front wheel with the underlying surface or road is asymmetrical. Basically, yaw torque is decreased by limiting the pressure rise on the wheel which last indicates a tendency to block (which is referred to as the "high wheel" or the high-brake pressure wheel) if the other wheel, the "low wheel" or lower brake pressure wheel, has the tendency to block.
Various solutions for decrease of yaw torques have been known, for example to permit pressure rise on the high-brake pressure wheel upon sensing tendency to block at the low-brake pressure wheel only at a decreased rate, for example by pulsing application of pressure rise brake fluid; or to maintain pressure constant on the high-brake pressure wheel when sensing tendency to block at the low-brake pressure wheel, until pressure is again raised at the low-brake pressure wheel, and then increase the pressure, in common, at a low rate, for example by pulsing brake pressure. Possibly, a small additional pressure decrease on the high-brake pressure wheel may be commanded, for example in advance of a phase of maintaining braking pressure constant.
The yaw torque brake pressure decrease thus permits on the high-brake pressure wheel a rise in brake pressure to the maximum value only with some time delay. This results in temporal delay of the resulting yaw torque, due to the limited maximum difference in brake effort to such an extent that the driver or operator will be given sufficient reaction time to react to the driving condition. The instant of time for the temporally limited brake pressure rise on the high-brake pressure wheel is derived from instability of operation, that is, rotary operation of the low-brake pressure wheel. Similar conditions may occur at the wheels at the outside of a curve and the inside of a curve when braking is commanded and while the vehicle is operating in the curve. Recognition of braking in a curve thus cannot be derived from the operating conditions of the wheels themselves.
Many vehicles have a tendency to over-steer, which means that, as the transverse acceleration increases as the vehicle passes through a curve, with only slight deceleration of the vehicle, the vehicle will have a tendency to take the curve sharper that is, to move towards the inside of the curve. A vehicle which has an ABS without yaw torque brake decrease or limiting will be similar to one without it, that is, the tendency to oversteer will be retained if the deceleration due to braking is low. If, however, the operator commands a higher braking effort, a torque will occur at the front wheel at the outside of the curve due to the increase in braking effort thereon, which is a torque which tends to counteract the torque towards the inside of the curve, thus tending to cancel the over-steering behavior of the vehicle.
Increase of braking effort at the front wheel at the outside of the curve is time-delayed in vehicles having ABS and yaw torque brake limiting; the counteracting torque, then, will build up in most instances too late, so that the vehicle will receive a substantial torque pulse in the direction of oversteering, that is, towards the inside of the curve. In spite of desired increase of delay by braking, the behavior when braking while the vehicle is passing through a curve will be related to the behavior similar to low vehicle deceleration. The advantage of the counteracting torque thus cannot be obtained.
It has been proposed to eliminate this disadvantage by disabling the effect of the yaw torque brake limitation when the vehicle passes through a curve. This can be obtained, by disconnecting or reducing the yaw torque decrease when the vehicle is operating in a curve, that is, when operation in a curve path is being sensed.